1. Field of the Invention
The present invention relates to a fault recovery system of a ring network, and more particularly to a fault recovery system of a ring network based on a synchronous transport module (STM) transmission system called a new synchronization system.
Ring networks based on the synchronous transport module (STM) transmission system such as a synchronous digital hierarchy (SDH) or synchronous optical network (SONET) the standardization of which has been developed in the CCITT or United States T1 Committee, are expected to be applied to subscriber systems (Urban Networks) in the future. The STM transmission system is applied to a high speed and broad band system of more than 155.52 Mbps. When a ring network, based on such a STM transmission system which is a high speed and broad band optical transmission system, is constructed, the ability to survive a fault in the network is important and should be considered from the beginning of the construction of the system, since a network fault can have a great influence on the transfer of information in a modern information society.
2. Description of the Related Art
As a conventionally proposed network fault recovery system, there are recovery systems employing a loopback used in a local area network (LAN) and so forth. These conventional recovery systems, however, are networks based on packet communication through predetermined protocols, and therefore, there are problems in that it takes a long processing time of several seconds to recover from a fault because the fault must be recovered by the use of the above-mentioned predetermined protocols. Since the recovery time in a new synchronization system should be shorter than, for example, 50 msec, a recovery method which uses these protocols cannot be employed in a new synchronization system.
On the other hand, for point to point communication, the standard usage of automatic protection scheme (APS) bytes (K1 and K2 bytes in an STM frame) for a switching control between a working line and a protection line has been recommended by the CCITT or the United States T1 Committee. For application to a ring network, however, standard usage has not been proposed.
A fault recovery system applied to a ring network is disclosed in Japanese Patent Publication 1-45782, published on Oct. 4, 1989. This fault recovery system, however, is not applied to the STM transmission system. Further, in this document, if multiple faults occur in the working line and the protection line, the positions of the faults cannot be determined.